This project would continue ongoing investigations of the neurobiology of visual attention in normal subjects and neurologic patients. These investigators' previous work has identified the neural regions associated with three elementary operations for allocating visual attention: the parietal lobe disengages, the midbrain moves, and the thalamus engages attention. A continuing focus of this project has been the role of midbrain extra-geniculate visual pathways in controlling visually guided behavior. Chronometric measures of detection reaction time and saccade latency are employed to obtain converging evidence about collicular function from three subject groups: (a) in progressive supranuclear palsy patients, the effects of collicular lesions are examined on orienting behavior; (b) in hemianopic patients, experiments determine what orienting behaviors are preserved when the only available visual input is from the retinotectal pathway; and (c) in normal human subjects, an asymmetry in retinotectal projections from nasal and temporal hemiretinae is exploited to compare the efficiency of orienting attention and eye movements into temporal and nasal hemifields. These studies have established that the retinotectal pathway is critical for moving attention and for favoring novelty in visual scanning by inhibiting reorienting to recently attended locations. The studies proposed would further examine the interplay between reflexive and controlled neural processes in the allocation of visual attention. The goals are: (1) to specify the role of human extra-geniculate pathways in orienting attention to visual signals; and (2) to determine how these subcortical mechanisms for reflex orienting are regulated by higher centers which govern voluntary, goal-directed behavior. The proposed experiments would test the hypothesis that midbrain visual pathways have a special role in orienting attention to exogenous signals and in integrating attention with eye movement control. Other experiments would investigate the role of prefrontal cortex and the basal ganglia in orienting attention under endogenous control. These studies would attempt to determine how midbrain mechanisms for reflex orienting are regulated by these higher centers, and whether the prefrontal cortex or basal ganglia have a special role in orienting under delayed response conditions and, if so, whether the mechanisms involved reflect spatial memory capacity, sustained attention to a location, or control of oculomotor set.